Hello Belgium,
Pin 11 (Backlight-ON) is out of action at all times at 0V. I would expect this would cause the SSB to trigger protection (blinking LED), after which +12V (pins 5 & 6) and 12.3V Sound (pins 7 & 8) are deliberately stifled.
Could only find double-digit codes in the service manual, so maybe you've misinterpreted the code. Can't put a suggested board to your code, but if the SSB at no time is able to supply BL-ON volts (would expect 3.5V required) and you are able to force backlights on, then I would say the SSB is the offender. Probably too complex to fault-find the SSB, so it's off to market for a replacement, but I suspect you knew this.
Bruce.